Les Principes de base de la modélisation des procédés de Mise en Forme des Polymères

National audienceDans le passé, le développement de nouveaux polymères était motivé par de nouvelles propriétés ou par des propriétés améliorées que l'on recherchait. On stipulait que les procédés de Mise en Forme s'adapteraient via quelques ajustements réalisés par une démarche d'essais-erreurs. Cela a fonctionné dans un certain nombre de cas, mais dans d'autres, on a découvert rapidement que ces nouveaux polymères, aux propriétés " merveilleuses " étaient très difficiles à mettre en oeuvre, soit parce que les pressions ou les couples nécessaires étaient incompatibles avec les machines de mise en oeuvre existantes, soit parce qu'ils présentaient des instabilités d'écoulement ou d'étirage à des vitesses de production incompatibles avec leur rentabilité économique. On se souvient à cet égard du développement des premiers Polyéthylènes metallocènes qui présentaient des défauts d'extrusion à des taux de cisaillement de l'ordre de quelques s-1. Plus récemment, les producteurs de polymères ont intégré le procédé dans le développement de leurs nouveaux matériaux en utilisant dés les premiers stades de leur développement des mini-machines de transformation (extrudeuses ou presses à injecter) qui permettent de discerner d'éventuels problèmes de mise en oeuvre avec quelques centaines de grammes de polymère. Malheureusement l'extrapolation à des machines de taille industrielle s'est révélée parfois hasardeuse , en particulier du fait que la " mise à l'échelle " n'obéit pas à la même homothétie selon que l'on considère les phénomènes mécaniques ou thermiques. C'est la raison pour laquelle la modélisation numérique, utilisée essentiellement dans le domaine de la conception des moules d'injection il y a une vingtaine d'années, pénètre aujourd'hui les différents secteurs de la plasturgie. Mais cela n'a un sens que dans la mesure ou les modèles numériques développés sont basés sur une analyse physique pertinente des phénomènes rencontrés dans une machine ou un outillage de mise en oeuvre. L'ambition de cet article est de donner quelques règles qui permettent d'adapter le modèle aux contraintes du procédé de mise en oeuvre et aux questions auxquelles on souhaite apporter une réponse, avant de les illustrer sur quelques exemples. Mais, tout d'abord, nous dressons un " état de l'art " rapide des procédés

An Overview of Molten Polymer Drawing Instabilities

International audienceDrawing instabilities and rupture are a serious limitation in polymer fibre and film processing. Draw resonance and fibre or film rupture depend on the processing conditions, heat transfer and on the rheology of the polymer and some of these defects may also be encountered for Newtonian fluids. This paper reviews the different instabilities observed in fibre spinning, cast-film and film blowing. The time dependent equations are presented for the simplified situation of constant width cast-film and two modelling strategies, linear stability analysis and direct simulation, are proposed and then applied to the different fibre and film processes. Read More: http://www.hanser-elibrary.com/doi/abs/10.3139/217.283

Technological solutions to reduce the geometrical defects during the cast film process

International audienceThe polymer cast film process induces some geometrical defects : the width reduction (neck-in defect) and the inhomogeneous decrease of the thickness distribution (edge bead defect). These defects prevent from predicting the film final dimensions and thus from a high industrial productivity. In this paper, a numerical model developed by the authors is used to test and optimize two technological solutions, aimed at reducing the geometrical defects : a localized air cooling on the edges of the film at the die exit and an "encapsulation die" which enables the extrusion of a high melt strength polymer along the outer edge of a core polymer. Numerical results are confronted with experimental investigation

Polymer rheology at high shear rate for microinjection moulding

International audienceIn this paper, we present the rheology of a high-density polyethylene for microinjection moulding. At high flow rate, pressure becomes high enough so that shear heating and pressure-dependence of the viscosity cannot be neglected. Moreover, spurt and chaotic defects related to wall slip were encountered. Nevertheless, we used classical data processing. We obtained viscosity at high shear rate which is lower than the viscosity obtained with the Carreau-Yasuda extrapolation of the moderate shear rate data. In order to discuss the origin of this difference, we developed a flow numerical simulation in the capillary where we take into account pressure and temperature dependent viscosity. The relative importance of these phenomena and their coupling is presented. Then, injection moulding experiments were carried out in a plaque mould. Different moulding parameters were tested. A three dimensional injection moulding numerical simulation, using Rem3D software, was carried out. The quality of the different sets of rheological data is assessed by comparing computed and measured pressure

A Personal Perspective on the Use of Modelling Simulation for Polymer Melt Processing

Copyright 2015 Carl Hanser Verlag. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the Carl Hanser Verlag. The authors are grateful to the publisher, Carl Hanser Verlag, for letting the manuscript being archived in this Open Access repository. The final publication is available at = http://dx.doi.org/10.3139/217.3020International audienceThis paper gives a personal view on the state of art in relation to the modelling of polymer melt processing. The paper briefly reviews both industrial, laboratory and modelling developments over the last forty years and highlights the key aspects now required for realistic modelling of polymer melt processing. The paper summarizes elements relating to the numerical simulation of specific and general polymer processes and also provides topical examples of the application of numerical modelling to certain commercial processes. The paper concludes with identifying areas of polymer processing that still remain a challenge in relation to accurate prediction

Numerical Investigations of Polyester Coextrusion Instabilities

International audienceInterfacial instabilities occurring in the coextrusion process of molten polymers have been widely studied. The theoretical work based on the stability of 2D Poiseuille multilayer flows (invariant along the flow motion) have pointed out the convective nature of this instability (it is either amplified or damped in the flow direction). This behaviour has been confirmed by experimental observations. As this instability is purely elastic (the Reynold number in coextrusion process is very small), it is necessary to introduce a viscoelastic constitutive equation for the polymers. Comparisons between experimental observations and theoretical approaches based on the Giesekus or White-Metzner models give good agreements for laboratory simple devices. However, the applicability of this approach for industrial coextrusion process remains an important challenge, because one has to deal with the influence of complex geometries, varying temperature and more realistic constitutive equations. As it is still very costly in computational time to perform 3D instationary computations for viscoelastic fluids, a methodology to analyse the appearance of interfacial instabilities in a complex geometry is proposed within this paper. First, a stationary 3D computation is performed in the real geometry for a single polymer. This computation allows to find zones in which the flow motion is essentially 2D, isotherm and with maximum values of the shear rate and, thus, the Weissenberg number. Instationary 2D finite element computations for two layer flows and a multi-mode differential viscoelastic model are then performed in these zones. Spatio-temporal analyses give information on the spatial behaviour of interfacial disturbances. This methodology is applied to the coextrusion of two polyesters for which experimental observations are available. The simulations predict well the experimental results and show that the outlet zone can amplify low frequencies perturbations. Finally, the influence of processing parameters (temperatures, flow rates, ...) and die geometries has been checked

Utilisation de méthodes de champ pour étudier les lois de comportement des polymères fondus

Colloque avec actes et comité de lecture. Internationale.On utilise la vélocimétrie laser et la biréfringence d'écoulement pour mesurer les champs de vitesse et de contraintes dans des écoulements de polymères fondus dans des géométries convergentes. La démarche choisie pour valider des lois de comportement rhéologiques consiste à, (a) calculer les vitesses de déformations à partir des mesures de la vitesse, (b) déduire les contraintes en utilisant une loi de comportement basée sur des théories macromoléculaires (Marrucci et Ianniruberto [1]), (c) confronter les contraintes calculées à celles mesurées par biréfringence d'écoulement, (d) itérer sur les valeurs des paramètres non linéaires de la loi choisie jusqu'à réaliser un accord satisfaisant. Dans un premier temps nous nous sommes intéressés aux phénomènes sur l'axe de l'écoulement

Injection of thermoset foam: comparison between simulation and experiment

International audienceThe quality (cellular homogeneity, mechanical properties) of polyurethane foam's structures mainly depends on the manufacturing process, during which two concomitant (principal) exothermic chemical reactions take place: the first one creates CO2 into the fluid matrix (germination of bubbles, expansion and coarsening of the foam) and the second one leads to the polymerization. In order to validate a model developed at CEMEF, an original experiment (RheoFoam System) has been created. It consists in an instrumented injection mould (closed or opened cylindrical cavity) in which the viscoelastic foam inflates. It allows measuring simultaneously the evolution of some technological parameters (the rise of the foam, the pressure distribution on the bottom of the mould and the temperature evolution inside the foam) which are a macroscopic signature of the evolution of the cellular microstructure. These temperature and pressure fields are then compared to those obtained using the numerical simulation. The results are discussed

Finite element and automatic remeshing methods for the simulation of complex blow molded polymer components

International audienceThis paper presents a three dimensional finite element model of the extrusion blow molding process. The code Tform3 has the following characteristics: membrane formulation, linear triangle elements, updated Lagrangian implicit formulation, viscoelastic differential constitutive equations. The paper presents a brief recall of the formulation and then addresses three key issues of the simulation: automatic identification of constitutive equation parameters, automatic remeshing, coupling between gas pressure and inflation. An example of application to the extrusion blow molding of a bottle is presented

Stability of cast-film extrusion of various metallocene polyethylenes

International audienceThe Cast Film process is one of the most widely used polymer processing technology to produce polymer films. A polymer melt is extruded through a flat die and then stretched in air by a chill roll to generate a thin film as illustrated in Figure 1. The molten film is quenched on the surface of the chill roll (typically water-cooled and chrome-plated) by an air knife or a vacuum box